Photon generator

ABSTRACT

The invention provides an apparatus for optical integrated on-chip generation of photon pairs as a building block to create entangled photon states required for quantum information processing. The invention provided a frequency selective optical coupling device which controls the transmission of light by varying the relative dimensions of otherwise symmetrical linear optical waveguides tangential to an annular optical waveguide, thereby controlling the coupling of light between the linear optical waveguides and the annular optical waveguide. Dimensional change of the optical waveguides is achieved by a heated medium in proximity of the optical waveguides and under electronic control.

PRIORITY CLAIM UNDER 35 U.S.C. § 119(e)

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application Ser. No. 62/424,739 filed on Nov. 21,2016, the entire content of which is incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the field of quantum informationprocessing and more specifically to integrated photonic devices thatfacilitate the same.

BACKGROUND OF THE INVENTION

Integrated photonics is proving to be a very promising platform forquantum information processing. Micro ring resonators are becoming a keycomponent of such systems as they have been shown to be effective asphoton-pair sources by means of exploiting a materials nonlinearity forspontaneous parametric downconversion (SPDC) or spontaneous four wavemixing (SFWM).

Often, it is desirable to have precisely one photon. While SPDC and SFWMsources generate pairs of photons, single photons can be achievedthrough heralding. Heralding is a technique in which the detection of asingle photon from a pair is used to determine the existence of theother. One of the fundamental issues with ring resonators is theirinherent 50% loss when critically coupled, regardless of operation in asingle bus or double bus configuration. For single bus resonators (notshown), half of the generated photons are lost to scattering within thecavity.

Referring to FIG. 1 depicts prior art double bus resonators which areslightly different as the photons are free to leave the ring 10 througheither port—resulting in an effective loss of 50%. All of this assumesthat the ring resonator is critically coupled to straight waveguides 20,30.

As with the two typical forms of ring resonators, they are denoted bythe number of waveguides which near them giving them the titles ofsingle bus and double bus, respectively. Both resonators work on thesame principle. When light after a full round trip around the ring is ofequal intensity and opposite phase to light that is reflecting into thering, there is a destructive interference and no light can leave theresonator. Running time in reverse and seeing the light from the ringsplit at the directional coupler is an equivalent way to view thiseffect. In the case of the single bus resonator with no loss, resonancecan only happen for a coupling ratio of 50/50 from the bus waveguide.When loss is present, this can happen for much lower splitting ratios.One form that loss can take is scattering. The double bus resonator canbe seen as a special case of the single bus resonator where thescattering is captured into the second waveguide.

When the ring resonator is used for generation of single photons, twopump photons are absorbed and two single photons of equal energy to thepumps are created. Consequentially, the single photon light which isgenerated inside of the cavity has no input light to interfere with.Still referring to FIG. 1, therefore, in the case of the double busresonator with the same coupler on input and output, the light has anequal probability of exiting the first 20 and second 30 waveguide buses.This splitting is witnessed as intrinsic loss. In the case of single busring resonators, the light can either leave through the input port or belost inside the ring. When the pump wavelengths are optimally coupled,the propagation losses around the ring balance with the coupling out ofthe ring. The generated single photons (like the pumps) will have thissame balance in terms of loss and ability to couple out of the ring. Inother words, the single photons leave the ring only 50% of the time. Theodds of the single photons leaving the ring can be improved at the costof how well the pump wavelengths are coupled. This is a compromisebetween loss and generation rate.

The underlying issue of single and double bus ring resonators is thatthey do not have wavelength discriminating couplers. It is wellunderstood there doesn't exist dichroic mirrors on a chip presently.Moreover, in 1995, Barbarossa found that resonant wavelengths of a microring cavity could theoretically be suppressed by coupling the inputwaveguide to the ring at two points. However Barbarossa's designprovided an optical filter for classical light without generating anyphotons in the resonator cavity. What is lacking in prior work andtherefore still needed is a device that generates entangled pairs ofphotons and interferometric coupling as a filter for quantum states oflight.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide anapparatus and method to generate entangled pairs of photons for use inquantum information processing.

It is another object of the present invention to provide an integratedphotonic apparatus and method that generates entangled pairs of photons.

In a fundamental embodiment of the present invention, a frequencyselective optical coupling device, comprises an annular optical channel,a first linear optical channel having a first input and a first outputwhere the first linear optical channel is substantially tangential tothe annular optical channel at a first point and a second point, asecond linear optical channel having a second input and a second output,where the second linear optical channel is substantially tangential tothe annular optical channel at a third point and a fourth point; and apredeterminable relative phase delay between the first and the secondlinear optical channels so as to cause a variance in an amount of lighttraversing the first and the second linear optical channels as afunction of the frequency of the light.

In the preferred embodiment of the present invention, a photon generatordevice comprises an annular optical channel disposed in a chip, a firstlinear optical channel disposed in the chip, where the channel has afirst input and a first output and where the first input and a firstoutput are in common with each other and with an input to the chip,where the first linear optical channel is substantially tangential tothe annular optical channel at a first point and a second point, andwhere a second linear optical channel is disposed in the chip with thesecond linear optical channel having a second input and a second output,where the second linear optical channel is substantially tangential tothe annular optical channel at a third point and a fourth point, a firstpredeterminable relative phase delay between the first and the secondlinear optical channels so as to cause a variance in an amount of lighttraversing the first and the second linear optical channels as afunction of the frequency of said light, and a second predeterminablerelative phase delay between the second input and the second output, aphoton detector sampling each of the second input and the second output,and a third output of the chip in common with the second input, a fourthoutput of the chip in common with the second output, and an electroniccontrol subsystem in operative communication with the chip forfacilitating the predeterminable relative phase delays and the photondetection.

Briefly stated, the invention provides an apparatus for opticalintegrated on-chip generation of photon pairs as a building block tocreate entangled photon states required for quantum informationprocessing. The invention provided a frequency selective opticalcoupling device which controls the transmission of light by varying therelative dimensions of otherwise symmetrical linear optical waveguidestangential to an annular optical waveguide, thereby controlling thecoupling of light between the linear optical waveguides and the annularoptical waveguide. Dimensional change of the optical waveguides isachieved by a heated medium in proximity of the optical waveguides andunder electronic control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art double bus resonator showing the couplingcoefficients to the two waveguides.

FIG. 2a is a Dual Mach-Zehnder device design of the present invention.

FIG. 2b is a microscope image of a fabricated Dual Mach-Zehnder deviceof the present invention.

FIG. 3 is a Dual Mach-Zehnder theoretical spectrum showing suppressedresonances at the input side.

FIG. 3b is a Dual Mach-Zehnder theoretical spectrum showing output sidetransmission.

FIG. 4 is a Dual Mach-Zehnder experimentally generated spectrum showingsuppressed resonances.

FIG. 5a is a Dual Mach-Zehnder measured photon pairs in the untunedconfiguration.

FIG. 5b is a Dual Mach-Zehnder measured photon pairs in the tunedconfiguration.

FIG. 6 is an embodiment of the present invention employing a DualMach-Zehnder used to produce energy-time entangled photonspairs/squeezed beams.

FIG. 7 is an embodiment of the present invention employing a DualMach-Zehnder used to produce N00N states or pairs/squeezed beams.

FIG. 8 is an embodiment of the present invention employing a DualMach-Zehnder used to produce N00N states and pairs/squeezed beamssimultaneously.

FIG. 9 is an embodiment of the present invention employing a DualMach-Zehnder used to produce N00N states, frequency combs, andpairs/squeezed beams simultaneously.

FIG. 10 is an embodiment of the present invention employing a DualMach-Zehnder used to produce N00N states, frequency combs, andpairs/squeezed beams simultaneously.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An object of the present invention is to devise a wavelength dependentmeans to split light. The present invention employs a Dual Mach-Zehnder(MZI) device having legs that are grossly misbalanced, wherein the MZIwill have a wavelength dependence to its ability to split light. Thepresent invention devises two unbalanced MZI, one which will perfectlytransmit the pump wavelengths and partially reflect the signalwavelength. The other MZI will do the opposite, reflecting the pumpwavelengths but perfectly transmitting the signal wavelength.

Referring to FIG. 2a and FIG. 2b , the present invention essentiallymakes a Mach-Zehnder interferometer (MZI) out of the input waveguide 40and the ring 50. Being a cavity, the ring 50 will only support specificwavelengths of light (where the resonance condition is satisfied)separated by the free spectral range (FSR). The spectrum of anunbalanced MZI is sinusoidal with the difference in optical path lengthbetween the two paths determining where in the spectrum the constructiveand destructive interference will occur. For both the ring and the MZI,this is known as phase-matching. For the case of the ring this isphase-matching between consecutive round-trips while in the MZI it isphase-matching between the two different paths. The points ofconstructive interference in the spectra of these devices can be tunedby adjusting the relative phase between the different paths. In afabricated device (see FIG. 2b ) this can be accomplished by heaters orelectro-optic phase shifters. The combination of these two elementsresults in a phase-matching condition that relies on both the resonancecondition of the ring 50 and the interference pattern of the MZI. If thespectral width between two wavelengths of constructive interference inthe MZI is twice the FSR of the ring 50, it is possible to suppressevery second resonance of the ring 50.

For the case of the photon-pair source function of the presentinvention, one side of the ring 50 can be used as the input 40 for thepump photons and the drop side 60 as the output for the generatedphoton-pairs. The MZI on the input side 40 (MZI1) can be tuned tosuppress every other resonance, while MZI2 on the output of the ring 50can be tuned to suppress the resonances allowed by MZI1 (i.e. they areperfectly out of phase with each other). This configuration will ensurethe pump laser is critically coupled into the ring 50 while not allowingit to exit out the drop port 60, and ensures that any photons that aregenerated at the resonances allowed by the drop port 60 will only exitthe over-coupled drop port (because MZI1 is tuned to not be phasedmatched with those photons). This makes the device function as though itis two independent single bus ring resonators, one for the input sideand one for the output side. The input side ring is characterized by thetransmission from the input port 40 to the through port 70 while theoutput side ring is characterized by the transmission from the add port80 to the drop port 60. The theoretical spectral response for both theinput and output sides are shown in FIG. 3a and FIG. 3b , respectively.This configuration has three key features: (i) The pump is criticallycoupled so the photon generation rate will be maximized; (ii) The pumpis filtered from the photons that exit the drop port minimizing noiseand reducing the amount of off-chip filtering required; (iii) The photonpairs will always leave out the same over-coupled drop port, yielding100% coincidence ratio, maximizing heralding efficiency.

The theory of operation of the present invention has been experimentallyproven as shown in FIG. 4. The invention exhibits all the cavityresonances when the thermal tuning has not been optimized. When thethermal tuning has been adjusted the undesirable resonances aresuppressed as shown in FIG. 4. This demonstrates the spectral filteringof the device, along with the field enhancement from the ring cavity,and the directionality of the desired output for the generated photonsshown in FIG. 5a and FIG. 5b . All aforesaid traits being useful forquantum information science applications.

With the confirmation of the dual Mach-Zehnder configuration as anoptimal design for the generation of photon pairs, larger photon pairstates, and higher squeezed states, the functional building block can beutilized to create entangled states when combined with other integratedwaveguide circuits.

Detailed below are five different implementations of the presentinvention for quantum information science applications. These are notthe only implementations that this device can be configured in for theseapplications. The invention as stated can be used to generate, photonpairs, entangled states, larger entangled states, and higher squeezedstates (for continuous variable applications). All embodiments of thepresent invention described below can be utilized to generate any ofthese mentioned photon states. Lastly another benefit of the inventionis that the source acts as filter for the pump light. This is an easyproblem to deal with in bulk optics, but in integrated circuits,removing the pump is difficult since high rejection filters are requiredon chip to deal with ˜10 orders of magnitude difference in pump tosignal power. The present invention takes care of a large portion ofthis filtering.

Referring now to FIG. 6 depicts a dual Mach-Zehnder (DMZ) source beingsingle or bi-directionally pumped from a continuous wave or pulsed lasersource (not shown) via an optical waveguide 90. The lower diagram inFIG. 6 through FIG. 9 depicts an overlay of the off chip electronics 160and its associated control lines 170 (dashed lines) to detection 140 andphase shifting 150 elements.

The pump photons interact in the micro-ring resonator cavity 100 andproduce signal/idler photons, which exit via the optical waveguides 110to the right of the micro-ring resonator 100. The signal/idler photonspass through phase shifters 120 which can compensate for length andtiming differences before hitting an optical tap 130 where a smallportion may be sent to a photodetector 140 to monitor the photons. Theother ports 180 allows the photon pairs/squeezed beams to pass to therest of the circuit on the integrated chip or leave off chip. The deviceis controlled by off chip electronics 160, with electrical control lines170 being depicted as dashed lines. Part of what the off chipelectronics 160 controls are the “heater” mechanisms 150. Heatermechanisms 150 designated in FIG. 6 through FIG. 10 as wide, solid blacksections comprise material that is placed alongside optical waveguidewithin the DMZ. When activated by the off chip electronics, the heatermechanisms 150 heat the adjacent optical waveguide, causing adimensional change in the optical waveguide. The optical dimensionalchange insofar as the optical waveguide length is affected will cause aphase shift for any light therein. The net desired effect is to alterthe relative optical lengths between the upper and lower waveguideswithin the DMZ.

Referring now to FIG. 7 depicts DMZ source being pumped bi-directionallyfrom a laser (continuous wave or pulsed) via an optical waveguide 90.The pump photons interact in the ring resonator 100 and producesignal/idler photons in both clockwise and counter-clockwise directions,thus producing path indistinguishable photons created in the ring 100.These photons then exit via the optical waveguides 110 to the right ofthe ring. The signal/idler photons pass through phase shifters 120 whichcan compensate for length and timing differences before simultaneouslyimpinging on a directional coupler 190. This coupler 190 mixes thephoton states producing an entangled state called a N00N state, or Nphoton, Zero, Zero, N photon state. The state exits the coupler 190 andpasses to an optical tap 130 where a small portion may be sent to adetector 140 to monitor the photons. The other ports 180 allows thephotons to pass to the rest of the circuit on the integrated chip orleave off chip.

Referring now to FIG. 8 depicts DMZ source being pumped bi-directionallyfrom a laser (continuous wave or pulsed) via an optical waveguide 90.The pump photons interact in the ring resonator cavity 100 and producesignal/idler photons in both clockwise and counter-clockwise directions,thus producing path indistinguishable photons created in the ringresonator 100. These photons then exit via the optical waveguides 110 tothe right of the ring resonator 100. The spectrally degenerate photonsare selected by an optical ring resonator filter 200 and pass throughphase shifters 120 which can compensate for length and timingdifferences before impinging on a directional coupler 190. This coupler190 mixes the photon states producing an entangled state called a N00Nstate, or N photon, Zero, Zero, N photon state. The state exists thecoupler 190 and passes to the rest of the circuit on the integrated chipor leave off chip. The photons that are not selected by the filter 200travel on a different waveguide, passing through a phase shifter 120 andthen hitting an optical tap 130. This resonant comb of other wavelengthscan be monitored with a photodetector 140 or passed to other circuitryto be utilized elsewhere. This source, shown in FIG. 8, can then produceN00N states, entangled frequency combs, and or squeezed statessimultaneously.

Referring to FIG. 9 depicts The DMZ source is pumped bi-directionallyfrom a laser (continuous wave or pulsed) via an optical waveguide 90.The pump photons interact in the ring resonator cavity 100 and producesignal/idler photons in both clockwise and counter-clockwise directions,thus producing path indistinguishable photons created in the ringresonator 100. These photons then exit via the optical waveguides 110 tothe right of the ring resonator 100. The spectrally degenerate photonsare selected by an optical ring resonator filter 200 and pass throughphase shifters 120 which can compensate for length and timingdifferences before impinging on a directional coupler 190. This coupler190 mixes the photon states producing an entangled state called a N00Nstate, or N photon, Zero, Zero, N photon state. The state exits thecoupler 190 and passes to the rest of the circuit on the integrated chipor leave off chip to other circuits. The photons that are not selectedby the ring resonator filter 200 travel on a different waveguide,passing through a phase shifter 120 followed by two additional filters210. These two secondary filters 210 can serve a number of functions.They can further filter the pump wavelength to allow for a filtered setof photons to leave on the original waveguide. They can each filter outa different set of wavelengths to produce more correlated outputs, oneon each set of filter outputs and letting the rest of the comb exit onthe original waveguide when multiple correlated outputs are required.This source can then produce N00N states, multiple energy-timecorrelated pairs/squeezed beams, entangled combs, and squeezed statessimultaneously.

Referring to FIG. 10 depicts the DMZ source being pumpedbi-directionally from a laser (continuous wave or pulsed) via an opticalwaveguide 90. The pump photons interact in the ring resonator cavity 100and produce signal/idler photons in both clockwise and counter-clockwisedirections, thus producing path indistinguishable photons created in thering resonator 100. These photons then exit via the optical waveguidesto the right of the ring resonator 100. The signal/idler photons passthrough phase shifters 120 which can compensate for length and timingdifferences before simultaneously impinging on a directional coupler190. This coupler 190 mixes the photon states producing an entangledstate called a N00N state, or N photon, Zero, Zero, N photon state. Thestate exits the coupler 190 and passes to a switching network which inone implementation could consist of Mach-Zehnder interferometers (MZI).These MZI's mix the photon states in a reconfigurable manner that allowsthe creation of entangled states. These can range from two photon (Bellstates) to larger entangled states (Cluster and Greene-Horne-Zeilinger(GHZ) states). The circuit can be used to produce states important forsmall scale quantum information processing, specifically quantumcomputation. The optical waveguides can terminate with photodetectors140 allowing the entire computation to be completed on chip.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

What is claimed is:
 1. A frequency selective optical coupling device,comprising: an annular optical channel; a first linear optical channelhaving a first input and a first output, said first linear opticalchannel being substantially tangential to said annular optical channelat a first point and a second point; a second linear optical channelhaving a second input and a second output, said second linear opticalchannel being substantially tangential to said annular optical channelat a third point and a fourth point; and a predeterminable relativephase delay between said first and said second linear optical channels,so as to cause a variance in an amount of light traversing said firstand said second linear optical channels as a function of the frequencyof said light.
 2. The frequency selective optical coupling device ofclaim 1, wherein said substantial tangentiality permits a coupling oflight between said annular optical channel and said linear opticalchannels at said first, said second, said third and said fourth points.3. The frequency selective optical coupling device of claim 1, whereinsaid predeterminable relative phase delay is induced by a relativedifference in length between said first linear optical channel and saidsecond linear optical channel.
 4. The frequency selective opticalcoupling device of claim 3, wherein said relative difference in lengthis induced by thermal expansion.
 5. The frequency selective opticalcoupling device of claim 4, wherein said thermal expansion is induced bya heated medium in proximity of said channels.
 6. A photon generatordevice, comprising: an annular optical channel disposed in a chip; afirst linear optical channel disposed in said chip, said channel havinga first input and a first output, said first input and a first outputbeing in common with each other and with an input to said chip; saidfirst linear optical channel being substantially tangential to saidannular optical channel at a first point and a second point; a secondlinear optical channel disposed in said chip, said channel having asecond input and a second output, said second linear optical channelbeing substantially tangential to said annular optical channel at athird point and a fourth point; a first predeterminable relative phasedelay between said first and said second linear optical channels, so asto cause a variance in an amount of light traversing said first and saidsecond linear optical channels as a function of the frequency of saidlight; a second predeterminable relative phase delay between said secondinput and said second output; a photon detector sampling each of saidsecond input and said second output; a third output of said chip incommon with said second input; a fourth output of said chip in commonwith said second output; and an electronic control subsystem inoperative communication with said chip for facilitating saidpredeterminable relative phase delays and said photon detection.
 7. Thephoton generator device of claim 9, wherein said substantialtangentiality permits a coupling of light between said annular opticalchannel and said linear optical channels at said first, said second,said third and said fourth points.
 8. The photon generator device ofclaim 9, wherein said first predeterminable relative phase delay isinduced by a relative difference in length between said first linearoptical channel and said second linear optical channel.
 9. The photongenerator device of claim 9, wherein said second predeterminablerelative phase delay is induced by a relative difference in the lengthof optical channel from said second input to said third output, and thelength of optical channel from said second output to said fourth output.10. The photon generator device of claim 11 or claim 12 wherein saidrelative difference in length is induced by thermal expansion.
 11. Thephoton generator device of claim 13, wherein said thermal expansion isinduced by a heated medium in proximity of said optical channels.